1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image formed on a sheet or recording medium conveyed to a nip for fixation, and an image forming apparatus using the same.
2. Description of the Background Art
An electrophotographic copier, printer, facsimile apparatus, multifunction machine or similar image forming apparatus often includes a fixing device of the type using a heat roller and a press roller. The heat roller or rotary body and press roller or pressing means are pressed against each other, forming a nip for fixation therebetween. When a sheet carrying a toner image thereon is brought to the nip, the heat roller being heated and press roller cooperate to fix the toner image on the sheet with heat and pressure.
Another conventional fixing device includes a fixing belt passed over a heat roller and a driven roller. A press roller facing the driven roller and belt form a nip for fixation therebetween. This type of fixing device insures sufficient pressure for fixation and maintains the nip stable.
It is a common practice with an image forming apparatus to shut off power supply to, e.g., a fixing heater when the apparatus is not used for a saving power purpose. The prerequisite with an energy saving type of fixing device using a heat roller is that the surface of the heat roller be immediately heated to temperature high enough to melt toner. To meet this prerequisite, it has been customary with a fixing device of the type disposing a halogen lamp in the heat roller to use a plurality of lamps for reducing the wall thickness of the heat roller to 1 mm or less and in consideration of a heat distribution.
Another conventional fixing device includes a planar heating member for heating a fixing roller and a resistance heating body formed on the heating member with the intermediary of an electric insulation layer. This type of fixing device has higher heat conversion efficiency and shorter warm-up time than the fixing device using lamps. A problem particular to such a rapid warm-up fixing device is that temperature rises at opposite ends of the heating member. However, this problem relating to a temperature distribution can be easily solved at low cost if the resistance heating body is divided into segments that can be controlled independently of each other. Generally, the resistance heating body extends in the axial direction of the fixing roller at the outside or the inside of the fixing roller. Power is fed to the resistance heating body via a terminal member affixed to the heating member, which extends as far as the opposite ends of the fixing roller. The terminal member contacts a metallic brush and slidingly moves in the circumferential direction. This type of fixing device is taught in, e.g., Japanese Patent Laid-Open Publication Nos. 62-200380 and 62-24288. Japanese Patent Laid-Open Publication No. 09-197853 discloses a fixing device that applies the planar heating member for heating the heat roller of a belt type fixing system. This fixing device also uses sliding contact for feeding power to a resistance heating body.
The fixing device of the type using a terminal and brush scheme has the following problems left unsolved. Because the resistance heating body (terminal) rotates, both the brush and terminal wear and produce dust and have surfaces roughened. This reduces the life of the brush and brings about noise due to spark discharge, lowering the reliability of the fixing device. Further, when the resistance heating body is divided in segments that are controlled independently of each other, the terminal portion becomes sophisticated due to the rotation of the heating bodies. The fixing device using a plurality of halogen lamps in consideration of a heat distribution is undesirable from the cost standpoint.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-262895, 8-335000, 9-212016 and 2000-131975.
It is an object of the present invention to provide a stably operable, long-life fixing device structurally free from wear dust and spark discharge, and an image forming apparatus using the same.
It is another object of the present invention to provide a stably operable, long life fixing device making most of the merits of a planar heating member and obviating wear dust and spark discharge ascribable to friction, and an image forming apparatus using the same.
In accordance with the present invention, a fixing device for fixing a toner image formed on a recording medium conveyed to a nip for fixation includes a rotary body, a stationary heating member not forming the nip, a fixing belt passed over the rotary body and heating member, and a pressing member held in contact with the fixing belt. A resistance heating body is formed on the heating member with the intermediary of an electric insulation layer.
An image forming apparatus using the fixing device described above is also disclosed.